A color display system utilizing a color cathode ray tube (CRT) is liable to gradual degradation of performance due to various parts gradually becoming magnetized. In particular, the shadow mask of a color CRT tends to become magnetized, causing electron beams to be deflected such that they no longer hit the front screen at the correct location and consequently fall on a region containing phosphor which emits an incorrect color. This leads to impurity in the color of the resultant image.
It is well known in the art that the amplitude of this magnetization may be reduced to an acceptable level by the use of a degauss coil, that is, a coil in the region of the CRT through which a decaying sinusoidal current is passed. Improved results can be obtained by using a pair of degauss coils on opposite sides of the CRT, thereby producing a more uniform degaussing field around the CRT.
Unwanted magnetism can also be generated outside the CRT itself; the two major sources are:
(1) the terrestrial magnetic field; and
(2) locally produced magnetism, e.g., from powerful electric motors
It is also desirable in high-quality color display systems of this type to compensate for these magnetic fields. Unfortunately, these cannot be eliminated by degaussing. Indeed, the problem may be exacerbated by degaussing since this magnetizes the ferromagnetic components of the CRT in a manner corresponding to the ambient magnetic field. Thus, after degaussing, the ferromagnetic components produce a magnetic field aligned with and adding to the ambient field.
Since the maximum amplitude of the vertical terrestrial component is approximately twice that of the horizontal terrestrial components, it is more important to compensate for the vertical component. Although there are techniques available for reducing both components of the terrestrial field, generally compensation is only provided for the vertical component.
An established technique for providing a fixed amount of compensation for the vertical magnetic component involves varying the position of the deflection coil yoke along the CRT neck. This is only feasible at the time of manufacture of the CRT.
There have been various attempts at compensating for terrestrial and other ambient magnetic fields once the display system is in use. One of these is disclosed in EP 77112 (Hazeltine) which describes a system for generating a magnetic field to counteract the component of the ambient field axial to the CRT (i.e. approximately horizontal) by the use of two coils, one on the viewing side of the CRT around its edge and the other parallel to the first but located behind the screen, towards the CRT yoke. These coils are permanently driven with a current generated in response to feedback signals from sensors at the corners of the screen which detect directly the beam landing error caused by the axial component of the terrestrial and other ambient magnetic fields. This method is moderately complex and expensive and prohibits the use of the entire screen for information display since the screen corners are obscured by the sensors.
Another compensation method is disclosed in EP 39502 (Siemens) in which three pairs of coils are arranged orthogonally around the CRT to compensate for all three orthogonal components of the ambient magnetic field. The signals to drive the coils are derived from orthogonally mounted Hall-effect sensors detecting the strength of the ambient magnetic fields in the three directions. The individual signal from each sensor is amplified and applied to the respective pair of coils. This method has the disadvantage that it is both complex to implement and expensive.
Yet another compensation method is disclosed in UK 1,493,311 (Sony). This is concerned with compensating for the two horizontal components of magnetic field. Instead of reducing the amplitude of some or all of the orthogonal components of the magnetic field over the majority of the length of the tube (as in the previous two examples), this technique utilizes a coil around the CRT yoke to produce a localized magnetic field to counterbalance the effect of the two horizontal components of the ambient field over the rest of the beam path (particularly through the CRT bell). This has the advantage that only one additional coil is required. However, it has the disadvantage that in the coil location used, it is necessary to drive the coil not with a simple constant direction current but with a complex double sawtooth waveform symmetrical about zero current. This makes this method expensive.
A similar compensation method is disclosed in JA 58-138191A (Mitsubishi Denki). This is concerned with compensating for all three orthogonal components of the ambient magnetic field and, like Sony, it uses an additional coil around the CRT yoke.